1. Technical Field of the Invention
The present invention relates generally to an alternator control apparatus designed to control generation of electrical power in an alternator for use in automotive passenger vehicles or trucks, and more particularly to an alternator control apparatus working to minimize an undesirable change in inertial torque of a rotor of the alternator.
2. Background Art
Most of modern automotive vehicles are so designed that many functions thereof are controlled electronically instead of mechanically. Electric power demands are, thus, increasing for various controls of the vehicles. This results in an increased size of automotive alternators serving as a power supply, which leads to an increased size of a rotor producing a magnetic field inside the alternator, thereby causing the moment of inertia thereof to be increased.
Additionally, an engine compartment is also required to be decreased in size for increasing the volume of a cabin. To this end, adoption of a serpentine belt drive system in which vehicle accessories including an alternator are driven through a single torque transmission belt is progressing in order to save a space in the engine compartment. Such a serpentine belt drive system uses an automatic belt tensioner (will also referred to as an auto-tensioner below) on a transmission line which absorbs a variation in tension of the belt to keep it constant. The use of such an auto-tensioner eliminates the need for a concern about a drop in tension of the belt with time. The tension of the belt is, thus, allowed to be set initially to a lower level.
There is a tendency to decrease the speed of an engine further during an idle mode of engine operation which does not contribute to driving the vehicle and is entered frequently for the sake of earth environmental protection such as decreases in exhaust emission and consumption of fuel. Moreover, a common rail fuel injection system working to elevate the pressure within a combustion chamber of diesel engines greatly is used increasingly for clearing exhaust gasses of the engine.
The above factors are usually combined, thus facilitating ease of occurrence of a variation in idle speed of the engine in synchronization with a power stroke of the engine. An essential problem is an increase in tension of the belt arising from a change in inertia torque of each accessory caused by the variation in idle speed of the engine. Particularly, automotive alternators, as described above, tend to increase the moment of inertia and are higher in pulley ratio than other accessories, which impinges upon the variation in tension of the belt greatly. In recent years, the accessories have increased their capacities and required great driving torques. The transmission of such a great driving torque to each accessory is preferably achieved with the serpentine belt drive system. The serpentine belt drive system requires an increased angle over which a belt is in contact with a pulley of each accessory and a compact layout of the pulleys within the engine compartment. In such a layout, an increased variation in tension of the belts will result in a great swing of the auto-tensioner, which may lead to collision with other accessories driven by the serpentine belt, thereby resulting in a decrease in service life of the auto-tensioner and the belt. Such a problem is posed notably, particularly in diesel engines because a variation in pressure within a combustion chamber is, as described above, great.
In order to avoid the above problem, Japanese Patent Second Publication No. 7-72585 and U.S. Pat. No. 4,725,259 teach use of a one-way clutch in a pulley of an automotive alternator which is designed to allow torque to be transmitted only in a single direction. Some advantages of the automotive alternator using such a one-way clutch will be described below in brief.
If a pulley having no clutch is used, the speed of the alternator decreases following a decrease in idle speed of the engine. The moment of inertia of a rotor of the alternator works as a motor to keep the speed of the alternator constant, thus loosening a rotation-stretched portion of the belt instantaneously, while stretching a rotation-loosened portion of the belt. The automatic belt tensioner, thus, swings to keep the tension of the belt constant.
If a pulley with a one-way clutch is used with the alternator, the clutch is disengaged by the moment of inertia of the rotor of the alternator when the speed of the engine is dropping (i.e., when the alternator works as a motor), and the speed of the rotor of the alternator is higher than the speed of the pulley, so that the speed of the pulley decreases slowly in synchronization with the speed of the engine. When the speed of the engine is increasing, the clutch remains disengaged until the speed of the pulley increases up to the speed of the rotor of the alternator. This avoids transmission of the inertia torque produced by the moment of inertia of the rotor of the alternator to the pulley, thereby decreasing the instantaneous decrease in tension of the stretched portion of the belt and the instantaneous increase in tension of the loosened portion of the belt, thus minimizing an undesirable change in tension of the whole of the belt.
Japanese Patent First Publication No. 6-207525 discloses a pulley with a bi-directional clutch. A coil spring which has an inner diameter greater than an outer diameter of a sleeve installed on a shaft of a rotor of the alternator is disposed between a pulley and the sleeve and joined at one end to the sleeve and at the other end to the pulley. The elasticity of the coil spring serves to absorb an instantaneous change in tension of the belt during a rise in speed of the engine as well as a drop thereof.
The one-way clutch, as taught in Japanese Patent Second Publication No. 7-72585, has a complex structure in which splines and a roller are engaged and disengaged selectively to establish and block transmission of torque between a pulley and a rotor, so that a large amount of stress is exerted on component parts of the clutch during connection or disconnection thereof. In design of such a type of one-way clutch capable of withstanding harsh conditions of use of the automotive alternator (e.g., a wide range of speed, acceleration, deceleration, and ambient temperature), the trade-off between reduction in size and increase in service life of the clutch is encountered. Specifically, in a case where the pulley ratio is increased to improve output of the alternator in a low-speed range of the engine, reduction in size of the pulley requires reduction in size of the clutch built in the pulley, thus resulting in a decrease in durability due to decreases in fatigue life and applied amount of grease. The manufacturing cost is also increased as compared with typical pulleys for ensuring desired accuracy of parts of the clutch and assembling thereof.
The bi-directional clutch pulley, as taught in Japanese Patent First Publication No. 6-207525, is so constructed that a load is exerted on the coil spring many times in a direction of rotation, thus posing a problem of durability of the coil spring and a spring support supporting ends of the coil spring. Increasing the rigidity of the coil spring to ensure desired durability requires increasing the size of the coil spring and the spring support, thus resulting in an increased size of the clutch pulley. This also makes it difficult to decrease the diameter of the pulley to increase the speed of thereof in order to increase an output power of the alternator meeting increasing electric power demands of automotive vehicles. In recent years, a clutch shoe has been proposed which is disposed between a coil spring and a pulley to decrease a mechanical load on the coil spring while improving the performance of the clutch in one direction. The clutch shoe works to slip to block transmission of the inertia torque of a rotor of the alternator during deceleration of the engine, but however, a difficulty is encountered in removing wear powder produced by the slip of the clutch shoe, which may cause the clutch shoe to be clogged with the wear powder, resulting in a premature decrease in performance of the clutch. The clutch still faces the problem of the increase in size.
It is therefore a principal object of the invention to avoid the disadvantages of the prior art.
It is another object of the invention to provide a control apparatus for an automotive alternator driven by an automotive engine which works to minimize a change in inertial torque of a rotor arising from a change in speed of the engine, thereby decreasing a change in tension of a belt hung over the alternator and the engine without sacrificing the durability of the alternator and increasing the size thereof.
According to one aspect of the invention, there is provided a control apparatus for an automotive alternator driven by an internal combustion engine installed in a vehicle which comprises: (a) an output voltage control circuit working to supply an exciting current to a rotor of the alternator so as to control an output voltage of the alternator; and (b) an inertial torque reducing control circuit working to perform a inertia torque reducing control which controls the exciting current flowing through the rotor so as to produce a power generating torque at the rotor which serves to reduce a change in inertial torque of the rotor arising from a change in speed of the internal combustion engine.
If a change in speed of the engine during an idle mode of engine operation is within a range of 40 to 100 rpm, a ratio of the speed of the alternator to the speed of the engine is three (3), the number of cylinders of the engine is six (6), and an average idle engine speed is 700 rpm, a frequency of a change in speed of the engine arising from a primary component of explosion will be 35 Hz. In this case, if the moment of inertia of the alternator is 3xc3x9710xe2x88x923 kgxc2x7m2, we have found that an inertial torque of 4 to 10 Nm is produced during a maximum acceleration of the engine.
A maximum driven torque of typical automotive alternators is several hundreds Nm. A change in tension of a belt hung over the engine and the alternator is, therefore, reduced by controlling the exciting current so as to produce the power generating torque which serves to reduce a change in inertial torque arising from the above change in engine speed, thereby producing substantially the same effect as that in a case where a pulley with a one-way clutch is used. Specifically, the above inertial torque reducing control may be performed easily in a slightly modified conventional controller without increasing production costs to minimize the change in tension of the belt with very high reliability and confidence levels without need for a concern about the durability of a clutch mechanism as used in the conventional system.
In the preferred mode of the invention, the inertia torque reducing control circuit includes a switch which works to control a supply of the exciting current to a fielding winding serving to magnetize field poles of the rotor, a speed determining circuit working to determine whether a speed of the internal combustion engine is lower than a first speed reference value or not, a first control circuit working to perform the inertia torque reducing control when it is determined that the speed of the internal combustion engine is lower than the first speed reference value, and a second control circuit working to perform an output voltage control which controls a switching operation of the switch based on the output voltage of the alternator when it is determined that the speed of the internal combustion engine is higher than the first speed reference value.
When the speed of the engine decreases below the first speed reference value, it results in a decrease in output torque of the engine, so that a change in speed synchronous with a power stroke of the engine becomes great. This causes the torque transmitted through the belt to be changed due to the moment of inertia of the rotor of the alternator, thus causing the tension of the belt to change. Such an event will disappears as the speed of the engine increases. Specifically, when the speed of the engine is low, the change in tension of the belt is decreased by reducing the inertia torque of the rotor.
The first control circuit may be designed to close the switch to supply the exciting current from a higher-voltage power supply designed to develop a voltage higher than a rated output voltage of the alternator when the speed of the engine exceeds a second speed reference value determined as a function of a change in speed of the engine. The first control circuit also works to open the switch to stop the supply of the exciting current and attenuates the exciting current flowing through the field winding using a circulating circuit line when the speed of the engine drops below the second speed reference value.
Typical automotive alternators have an inductance of several hundreds mH and uses a field winding having an electrical resistance of several xcexa9 in order to develop a required electromotive force with a small exciting current. The time constant of the field winding constituting an exciting circuit is, therefore, approximately 100 msec. This makes it impossible to control the torque by supplying the exciting current in a cycle of the above described 35 Hz engine speed change (i.e., 29 msec.) when the alternator provides a maximum output.
In order to avoid this problem, the higher-voltage power supply is used to supply the voltage higher than the rated output voltage of the alternator to the rotor so that a rated exciting current may flow through the field winding within a short time. For instance, if an exciting voltage of approximately 36V, as shown in FIG. 3, which is three times a normal rated output voltage is applied to the field winding, the exciting current reaches I0 near the rated exciting current 14 msec. (i.e., half of 35 Hz) after the field winding starts to be excited.
The exciting current If is expressed by
xe2x80x83If=(Vf/Rf)xc2x7(1xe2x88x92exp(xe2x88x92t/xcfx84))
where xcfx84 is the time constant which is given by Lf/Rf, Vf is the exciting voltage, and Rf is the resistance value of the field winding.
After the exciting current reaches I0, the switch is opened to attenuate the exciting current through the circulating circuit line. When the speed of the engine exceeds the second speed reference value (e.g., an average of the engine speed), the switch is closed, while when the speed of the engine drops below the second speed reference value, the switch is opened, thereby producing the power generating torque which serves to decrease the change in inertia torque of the rotor in order to minimize the change in tension of the belt.
The circulating circuit line may have disposed therein a resistor which has a resistance value of at least ten times an electrical resistance of the field winding. This facilitates the attenuation of the exciting current after the supply of the exciting current is stopped.
The exciting current If after the supply of the exciting current is stopped is expressed as
If=(I0+Vd/(Rf+Rb))xc2x7exp(xe2x88x92(Rf+Rb)xc2x7t/Lf)xe2x88x92Vd/(Rf+Rb)
where Rb is the resistance value of the resistor, and Vd is an on-voltage in the circulating circuit line.
The circulating circuit line may have disposed therein a rectifying device which has a forward on-voltage that is higher than a rated output voltage of the alternator. This provides the same effect as that in a case where a resistor having a higher resistance value is used, thus permitting the size of the control apparatus to be reduced and resulting in improved reliability thereof. The rectifying device may be made up of a plurality of diodes connected in series or a combination of a normal diode and a zener diode which are so connected in series as to have opposite polarities.
The exciting current If after the supply of the exciting current is stopped may be expressed as
If=(I0+Vb/Rf)xc2x7exp(xe2x88x92Rfxc2x7t/Lf)xe2x88x92Vb/Rf
where Vb is the on-voltage of a circulating device (consisting of the rectifying device and a circulating diode).
The field winding of the alternator may have a time constant of several msec. In this case, the first control circuit supplies the exciting current from a lower-voltage power supply designed to develop a voltage substantially identical with a rated output voltage of the alternator during a deceleration time when the speed of the engine decreases. The first control circuit opens the switch to stop the supply of the exciting current and attenuates the exciting current flowing through the field winding through a circulating circuit line during a deceleration time when the speed of the engine decreases. The use of such a high-response exciting circuit results in greatly improved response rate of the exciting current. Thus, during the deceleration time when the speed of the engine decreases, that is, when the moment of inertia of the alternator works as a motor, the switch is closed to decrease the inertia torque of the rotor through the power generating torque, while during the acceleration time when the speed of the engine increases, the switch is opened to avoid addition of the power generating torque to the inertial torque. This results in a decreased change in torque, thus minimizing the change in tension of the belt.
The control apparatus may further comprise a speed change rate determining circuit which determines a change rate of the speed of the engine. The first control circuit works to change a duty cycle of a signal used to turn on and off the switch as a function of the change rate of the speed of the engine as determined by the speed change rate determining circuit during the deceleration time. The determination of the change rate of the speed of the engine results in ease of determination of the deceleration time when the speed of the engine decreases, thereby facilitating ease of control of the power generating torque by the supply of the exciting current in synchronization with the speed of the engine. Changing the duty cycle as a function of the change rate of the speed of the engine permits the power generating torque to be change smoothly, thus enhancing the effect of reducing the change in inertia torque of the rotor.
The inertia torque reducing control may be performed on the alternator which is driven mechanically by an output torque of the engine through a belt. This minimizes an undesirable change in tension of the belt, thereby decreasing a rise in temperature of the belt due to slippage thereof and wear of the belt.
The alternator may be joined mechanically to the engine through a serpentine drive system which has a tensioner (e.g., automatic belt tensioner) working to keep tension of the belt constant. The use of the inertial torque reducing control with the serpentine drive system minimizes undesirable movement of the automatic belt tensioner, thus avoiding mechanical interference of the automatic belt tensioner with other vehicle accessories. This also permits the tension of the belt to be decreased, thus resulting in increased service life of the automatic belt tensioner and rotary parts supports (e.g., bearings) of vehicle accessories driven by the serpentine drive system.
The control apparatus may further comprise a speed determining circuit which works to determine the speed of the engine using the quantity of electricity as a function of frequency of a generated power of the alternator. This eliminates the need for a separate speed sensor for determining the speed of the engine, thus resulting in a decrease in manufacturing costs of the control apparatus.